It has long been the desire to produce an improved audio speaker, i.e., one that effectively reproduces the input waveform without distortion over a wide frequency range. Audio speakers, or electro-acoustic transducers, are frequently called on to reproduce an input waveform without distortion over a wide frequency range. In general, acoustic speaker systems include a current-carrying conductor, most commonly a coil, that reacts to the flux of a magnet in the motor by axially moving in response to the amount of current in the coil, i.e. the Lorentz force B×I. In general, as the coil moves, it drives a diaphragm, which produces a sound as a vibration in the air.
Distortions in the reproduced (or “transduced”) waveform may arise from a number of causes, including non-linear “motor force.” The motor force of a voice coil motor is referred to as “BL,” which is the magnetic flux density B times the effective length of the wire L in the magnetic field. BL is proportional to motor strength per unit of current—it generates a force of B×L×I. In general, the more constant and flat the BL curve, the more linear the motor and the lower the distortion.
If the coil moves outside the flux of the magnetic circuit, the B field interacting with the current in the coil may be reduced, leading to non-linear motor force. This non-linearity may reduce the axial driving force generated and create movement inconsistent with the desired waveform. This distortion tends to be exacerbated at the lowest frequencies, where large excursions become necessary to produce sound at an acceptable sound pressure level (“SPL”) or acoustic volume level. Indeed, the displaced volume required for a given SPL scales as the inverse square of the frequency (Vdα1/f2), thus requiring a driver to travel four times as far to reproduce a signal at half the frequency. Distortions in the reproduced waveform are minimized when the axial driving force remains constant over the required excursion.
Likewise, the inductance of a coil of wire can induce distortion in the transduced waveform by reducing the current of high frequency signals flowing through the coil. Inductance is proportional to the length of a coil, and rises as the frequency of the driving signal rises. The coil's impedance varies along with the inductance of the coil. In many cases, this rising impedance causes an increasing loss of axial driving force at higher frequencies, which distorts the signal by increasingly removing the upper frequency components, altering both the shape of the waveform and the frequency response. In some cases, the structure of the motor may cause excursion of the coil to modulate its inductance by position, causing an additional intermodulation distortion between low and high frequencies. In many cases, lowering inductance of a motor is preferable. Similarly, in many cases, the modulation of that inductance with position should be minimal.
Distortion in the reproduced waveform can also arise out of the voice coil's own magnetic field as it interacts with the motor's stationary magnetic field. In operation, a voice coil produces a voice coil magnetic field that is directly proportional to the amplitude of an applied speaker signal. The voice coil magnetic field affects the stationary magnetic field across the air gap in at least two ways. First, the voice coil magnetic field may weaken the stationary magnetic field by an amount that is proportional to the amplitude of the driving signal. As the driving signal increases and decreases in amplitude, the voice coil magnetic field modulates the stationary magnetic field, which in turn modulates the axial driving force, causing distortion.
One method of maintaining a flat BL curve at high excursions is taught by U.S. Pat. No. 7,039,213 to Hyre and Wiggins (hereinafter “Hyre”), which is incorporated herein by reference in its entirety. Hyre teaches an electro-mechanical transducer comprising a magnetic assembly that produces a magnetic field having two or more axially displaced regions of greater intensity (generally referred to as “gaps”), the displaced gaps being substantially similar in size, magnitude, and direction, and being separated by and surrounded by regions of lower intensity magnetic field. Hyre teaches that such displaced gaps may be achieved by including opposing grooves in the central pole and the top plate past which the coil moves to transduce sound.
Another method to minimize rising voice coil inductance is to use a fixed multi-turn coil in the gap (a “counter coil”), the counter coil producing a counter magnetic field that reduces the effect of the voice coil magnetic field on the stationary magnetic field. As a result, the counter coil reduces modulation of the stationary magnetic field by the voice coil magnetic field when subjected to the high amplitude speaker signals. Such counter coils are typically made out of multiple turns of wire wound around the pole piece and may be connected in series or in parallel with the voice coil. U.S. Pat. No. 2,004,735, granted to Thomas Jun. 11, 1935, discloses an actively-energized coil to neutralize changes in the gap flux density caused by variations in the field of the voice coil.
It is commonly known that the strength of the stationary magnetic field is inversely proportional to width of the air gap in the motor. Accordingly, it is known to be desirable to minimizing the width of the air gap, thereby strengthening the stationary magnetic field. However, a disadvantage to many previous counter coil implementations is that the air gap must be made wider to accommodate the counter coil.